I have been recently installed Zimbra behind HaProxy. HaProxy will load balancing for two MTA.

I have configured front-end & backend on HaProxy as you can see the below. I also configured on Outlook to send test email-message. All tests completed successfully. But, I have the problem while I'm sending a message from Outlook, it takes too much time by sending a message in about 5-7 seconds. So, can anyone else please help me give a few ideas or any solution to impair it.